The present invention relates to an antenna device and, more particularly, to an improved device adapted for attaining a sufficiently high receiving sensitivity in a mobile communication apparatus or the like.
In a mobile communication apparatus, it has been demanded heretofore the achievement of a capability of receiving a desired wave signal from any unknown direction of transmission.
For this purpose, a variety of techniques are proposed currently to realize satisfactory reception of a desired wave signal.
One of such known techniques is a space diversity receiving system which employs a plurality of antennas and selectively uses the antenna considered to have the best receiving sensitivity with regard to the desired wave signal.
FIG. 11 is a block diagram showing a structure of a conventional antenna device based on a space diversity receiving system.
In the diversity antenna 100 shown in FIG. 11, there are employed two antenna elements ANT1 and ANT2. These antenna elements ANT1 and ANT2 are so arranged that either one antenna element ANT1 or the other antenna element ANT2 that is considered to have a higher receiving sensitivity to the desired wave signal is selectively connected to a receiver 102 via a changeover switch 101. And, after selection of the signal of a desired frequency band in the receiver 102, the signal obtained from the receiver 102 is processed in a predetermined manner in a signal processor 103 and then is outputted therefrom.
However, it is impossible in the diversity antenna 100 of FIG. 11 to perform the control of antenna directional characteristics. That is, since the antenna beam direction cannot be turned to the direction of arrival of a desired wave signal, it is considered impossible to form the antenna beam optimally for reception of the desired wave signal. Therefore, depending on the direction of arrival of the desired wave signal, a sufficiently high receiving sensitivity fails to be attained to raise consequently a problem that some other interference wave signals than the desired wave signal are also received.
In view of the above problem, there is proposed an antenna device termed an adaptive array antenna based on an array antenna system which employs a plurality of transmission/reception lines and utilizes an adaptive signal processing so as to be capable of controlling the antenna directional characteristics.
FIG. 12 is a block diagram showing a structure of an adaptive array antenna.
In the adaptive array antenna 110 of FIG. 12 also, there are employed two antenna elements ANT1 and ANT2.
In this case, the antenna element ANT1 is connected to a receiver 102, wherein radio waves of a desired frequency band are selected and outputted to a signal processor 103. Meanwhile the antenna element ANT2 is connected to a receiver 105, wherein radio waves of a desired frequency band are selected and outputted to the signal processor 103.
In the signal processor 103, the received signals inputted from the receivers 102 and 105 are weighted respectively, and after predetermined signal processing such as combining is executed, the desired wave signal is obtained.
As for the antenna device capable of controlling the antenna directional characteristics, there is further proposed an array antenna device including feed elements therein, such as an Electronically Steerable Passive Array Radiator Antenna (hereinafter referred to as ESPAR antenna) (as disclosed in Patent Document 1).
FIG. 13 is a block diagram showing a structure of such an ESPAR antenna.
In the ESPAR antenna 120 of FIG. 13, there are also employed two antenna elements ANT1 and ANT2. In this case, the antenna element ANT1 is connected to a receiver 102, wherein radio waves of a desired frequency band are selected and outputted to a signal processor 103. Meanwhile, the antenna element ANT2 is connected to a reactance element 106, whose reactance value is controlled by the signal processor 103.
[Patent Document 1]
Japanese Patent Laid-open No. 2001-24431
However, in the adaptive array antenna 110 shown in FIG. 12, there exists the necessity of employing many antenna elements to constitute the required structure for attaining sufficient directional characteristics of the antenna. Therefore, it has been difficult heretofore to adopt such antenna in a mobile communication apparatus, particularly in a mobile terminal device where the number of antenna elements is limited due to physical conditions in installation.
Further regarding the ESPAR antenna 120 shown in FIG. 13, although the directional characteristics thereof may be controlled by providing at least two antenna elements, it also has been necessary to employ many antenna elements for constituting the required structure where sufficient antenna directional characteristics are attainable in a mobile communication apparatus.